Several different types of fuel cells comprise components made from ceramic materials, because ceramic materials are particularly robust during high temperature operation at and above 700° C. Ceramic materials are used in protonic ceramic fuel cells (PCFCs), molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs).
However, the primary disadvantage of current high temperature fuel cell technology is durability. The high temperatures at which these fuel cells operate accelerate component breakdown and corrosion, and therefore lead to decreasing fuel cell life.
SOFCs are solid-state devices which use an oxygen ion conducting ceramic electrolyte to produce electrical current by transferring oxygen ions from an oxidizing gas stream, such as air or oxidant, at the cathode of the fuel cell to a reducing gas stream, such as hydrogen, methane, natural gas, pentane, ethanol, or methanol, at the anode of the fuel cell. The SOFC, operating at a typical temperature between around 700° C. and around 1000° C., enable the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ions combine with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapour and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between the anode and the cathode, resulting in an electrical current flow through the circuit.
In order to increase the available power output of a fuel cell module, individual fuel cells are connected together in series to form a fuel cell bundle. Fuel cell bundles may be connected to adjacent fuel cell bundles to form a fuel cell strip. Fuel cell strips are connected together in parallel to form a fuel cell stack and multiple stacks may be connected together to form a module, to aggregate power output.
Fuel cells may be bundled together in the form of a plurality of planar elements, planar or cylindrical tubes, or other geometries. Fuel cell stacks, particularly those with planar geometry, often use seals between the electrolyte and the interconnecting surfaces to contain fuel and air at various locations within the fuel cell stack.
A typical fuel cell module may include a plurality of fuel cell stacks, the stacks may comprise a plurality of fuel cell strips connected in parallel, the fuel cell strips may comprise a number of fuel cell bundles and the bundles may comprise a number of fuel cell tubes or tube sub-assemblies.
Cracks in the ceramics used in the fuel cells or other components are a primary cause of failure of these devices. The thermal and mechanical loads imparted to the delicate fuel cells may lead to catastrophic failure of the fuel cells. Currently, fuel cell stacks undergo detailed inspection process including electrical measurements as well as visual inspections to detect cracks as early as possible in the manufacturing process. However, cracks within the fuel cells and within fuel/oxidant manifolds are known to manifest during the lifetime of a fuel cell stack.
EP0668622B1 discloses a solid oxide fuel cell, which comprises a plurality of modules. Some of these modules comprise hollow members, which have two parallel flat surfaces upon which the solid oxide fuel cells are arranged. The opposite ends of each module are connected to reactant manifolds by compliant bellow connections.
EP1419547B1 discloses a solid oxide fuel cell stack, which comprises a plurality of modules, the modules comprising elongate hollow members, the hollow members having a passage for flow of reactant. The modules are arranged so that at least one end of each module is connected to an end of an adjacent module to allow reactant to flow sequentially through the modules in a serpentine type arrangement.